The present invention relates to refractory castable compositions characterized by high abrasion resistance, high strength, and relatively low thermal conductivity.
Refractory castables are hydraulic setting compositions consisting of granular refractory aggregates and a refractory cement binder. Such materials are shipped as dry mixes to the location where they are to be used and then mixed with sufficient water to bring the mix to the desired consistency. The amount of water depends on whether the mix is to be cast, tamped, rammed, troweled, or gunned into position The reaction between the water and the cement additive develops a strong set at ambient temperature and later, as temperature is elevated such as in any furnace environment where the castable is applied, a strong ceramic bond develops as the cement bond further reacts with the refractory aggregate.
Such castables are particularly well suited for furnace linings having irregular contours.
One such application is the application of castables in connection with fluid catalytic cracking units and fluid coking units which are utilized by the petroleum industry to crack oil into a variety of products. In such units and transfer lines connected thereto highly abrasive catalysts or coke travel at high speed and cause erosion, in some cases extreme erosion, of the castable linings used. Therefore, one of the foremost requirements of a refractory castable for service in these units is abrasion resistance.
High abrasion resistance has been achieved by adding a hard, dense aggregate to the castable mix or by selecting certain fines which, when heat treated with the cement, form a hard, abrasion-resistant matrix. Another method of obtaining abrasion resistance is to use relatively large amounts of refractory cement which, when cured and fired, results in a hard bond.
Because of the relatively low temperatures involved in such petrochemical processes and the large expense of a fuel source, the need exists to conserve heat within the catalytic crackers This is usually accomplished by selecting refractory materials with low thermal conductivity. However, there is this difficulty of high resistance to abrasion. The materials which are highly abrasion resistant tend to be dense and dense materials inherently have high thermal conductivity. While both density and thermal conductivity may be lowered by adding a lightweight aggregate to the mix, the lightweight aggregates tend to be porous and poorly bonded and, hence, they have poor abrasion resistance.
Efforts to overcome that problem include utilizing amorphous silica with a calcium aluminate cement and a calcined high density refractory fine having a density of at least about 140 lb/ft.sup.3 as set forth in U.S. Pat. No. 4,656,146. While generally satisfactory, such compositions still do not have the degree of strength combined with low thermal conductivity that are required in such petrochemical processes.